This invention concerns surgical instruments and procedures and, in particular, systems, devices and methods for performing laser surgery (e.g., angioplasty) to treat atherosclerosis and the like.
Atherosclerosis is a disease which causes the thickening and hardening of the arteries, particularly the larger artery walls. It is characterized by lesions of raised fibrous plaque which form within the arterial lumen. The plaques are most prevalent in abdominal aorta, coronary arteries and carotid arteries and increase progressively with age. They commonly present dome-shaped, opaque, glistening surfaces which bulge into the lumen. A lesion typically will consist of a central core of lipid and necrotic cell debris, capped by a collagen fibromuscular layer. Complicated lesions will also include calcific deposits and various degrees of necrosis, thrombosis and ulceration.
The deformities of the arterial lumen presented by the plaque and associated deposits result in occluded blood flow, higher blood pressure and ultimately ischemic heart disease, if untreated. In 1984, coronary atherosclerosis was still the leading cause of death in the United States, claiming the lives of over a half million Americans annually, roughly twice as many as are killed by cancer.
The treatment of coronary atherosclerosis presently consists of drug therapy, thoracic surgery and percutaneous angioplasty. Drug therapy is primarily directed to the control of hypertension with diuretics, antiadrenergic agents, vasodilators and angiotension blockers. The goal of the drug therapy is to return the arterial pressure to normal levels and thereby reduce the stress on the patient's heart, kidneys and other organs. Unfortunately, drug therapy is not without side effects and cannot be relied upon to control progressive or acute atherosclerosis.
In the more serious instances of coronary atherosclerosis, thoracotomies are typically performed and so called "bypass" operations are conducted. In the bypass procedure, a vein (usually from the patient's leg) is utilized to construct a detour around the occluded coronary artery. One end of the vein is attached to the aorta, and the other end is attached to occluded vessel just beyond the obstruction. Although bypass surgery has become an accepted surgical procedure, it presents substantial morbidity risks, involves costs ranging from $25,000 to $40,000 in 1984 dollars, and generally requires extended hospitalization. Moreover, the procedure is often limited to arteries proximal to the heart and the long-term prognosis is less than satisfactory. Roughly 5 percent of the bypass grafts can be expected to occlude with each year following the operation; during this time it is not uncommon for the native artery to also become completed occluded as well, necessitating repeated procedures.
Recently, small balloon-tipped catheters have been developed which can be passed percutaneously into various arteries and inflated to dilate areas of partial obstruction. While this procedure has gained a measure of acceptance as a less invasive alternative to bypass surgery, balloon angioplasty simply redistributes the atherosclerosis stenoses; the frequency of reoccurence may be as high as 30 percent and such reoccurences further increase both as a function of the number of lesions treated and the time post-angioplasty.
Laser therapy has been suggested as another approach to angioplasty. For example, in a proposed procedure a catheter carrying a fiber optic waveguide is passed into an occluded blood vessel, positioned proximal to an atherosclerstic lesion and activated to decompose the plaque. Devices along these lines have been disclosed, for example, in U.S. Pat. No. 4,207,874 issued to Choy on June 17, 1980; and U.S. Pat. No. 4,448,188 issued to Loeb on May 15, 1984. See also generally, Marcruz et al., "Possibilidades Terapeuticas do Raio Laser em Ateromas," Vol 34, No. 9, Arq Bras Cardiol, (1980); Lee et al., "Laser Dissolution of Coronary Atherosclerosis Obstruction, Vol. 102, Amer Heart J, pp. 1074-1075 (1980); Abela et al., "Effects of Carbon Dioxide, Nd:YAG, and Argon Laser Radiation on Coronary Atherosclerosis Plaques,", Vol. 5, Amer J Cardiol, pp. 1199-1205 (1982); Choy et al., "Transluminal Laser Catheter Angioplasty" Vol. 50, Amer J Cardiol, pp. 1206-1208 (December 1982); Choy et al " Laser Coronary Angioplasty: Experience with 9 cadaver hearts," Amer J Cardiol, Vol. 50, pp. 1209-1211, (1982); Ginsberg et al., "Salvage of An Ischemic Limb by Laser Angioplasty, Description of a New Technique," Vol. 7, Clin Cardiol, pp. 54-58 (1984); Isner and Clark, "The Current Status of Lasers in the Treatment of Cardiovascular Disease", Vol. QE-20, No. 12, IEEE J Quantum Electronics, pp. 1406-1414 (1984); and Abela et al., "Laser Recanalization of Occluded Atherosclerotic Arteries In Vivo and In Vitro," Vol. 71, Circulation pp. 403-422 (1985), the teachings of which are incorporated herein by reference.
At present the use of laser angioplasty almost entirely has been restricted to animal studies and in vitro experiments on vessels obtained from human donors post mortem. The few reports of human therapy appear to confirm the feasibility of the procedure but the patency of arteries recannalized using present laser therapy techniques remains to be proven. A number of difficulties and adverse side effects also have emerged from the studies to date. Many of the attempts on excised blood vessels have resulted in charred tissue, coagulation necrosis and/or polymorphous lacunae. These pathological injuries suggest that blood vessels treated by laser angioplasty will require significant healing time periods and may be left with scarred, thromobogenic surfaces. Moreover, two other serious problems with present techniques are thermal and mechanical perforation. These perforations occur most commonly in connection with calcific deposits, branch points and torturous coronary arterial segments. Branch points and torturous coronary segments lead to mechanical and thermal perforations when they cause the optical fiber not to be coaxial with the artery.
Several histopathologic features of atherosclerotic arterial segments contribute directly to the problem of mechanical-thermal perforations. First, collagen is a principal component of atherosclerotic plaque. Because collagen typically imparts a white hue to the intimal surface of the plaque, the output of at least one commercially available laser system, the argon laser with its 454.514 nm blue-green light, is not preferentially absorbed. Second, calcification of the plaque further diminishes absorbance. Consequently, when the optical fiber initiates vaporization of plaque, the fiber will often "track" away from the calcified, severely fibrotic portions of the plaque toward the "softer" portions of the plaque, such as foci of yellow pultaceous debris or well developed (red) vascularity. These sites constitute a potential path of least resistance, which not infrequently promotes eccentric fiber-penetration of plaque into the highly absorbant (red) media and then, the outer adventitia. The result is perforation. Third, because the principal component of the media, smooth muscle, is characteristically depleted or attentuated in segments of atherosclerotic coronary arteries, a limited margin for error exists between the target of vaporization (i.e., plaque) and the underlying arterial wall.
There exists a need for better methods and devises for performing angioplasty. A system which could selectively remove complicated plaque lesions and associated materials from the arterial lumen with minimal injury to the underlying tissue and less risk of thermal or mechanical perforation would represent a substantial improvement. A system suitable for use in a surgical environment, with its components sealed to patient exposure and set to operate within a predefined range of optimal conditions, would satisfy a significant need in the art.